In recent years, a separation membrane such as microfiltration membrane or ultrafiltration membrane is utilized for a process in various areas including food industry and medical fields, water production and wastewater treatment fields, etc., because it has features of energy saving and space saving and in view of separation capacity, has characteristics of providing power saving, product quality enhancement, etc.
On the other hand, when membrane separation is applied to raw water, a membrane-impermeable substance such as suspended substances (hereinafter sometimes referred to as “suspended solids”) and organic matters contained in the raw water gradually sticks and deposits on the membrane surface or in a membrane pore to cause clogging of the separation membrane. As a result, the liquid flow resistance of the separation membrane is increased, and it eventually becomes impossible to perform membrane separation.
In order to eliminate clogging and restore the membrane separation performance, chemical cleaning of a separation membrane is generally conducted, but when suspended solids remain accumulated, the cleaning effect of the chemical solution is reduced. Alternatively, there is a case where the membrane separation performance can be restored by repeating cleaning with a chemical solution, but the amount of chemical solution used and the cleaning time are increased, resulting in a problem of suffering from a disadvantage in view of treatment cost.
Accordingly, with an attempt to continuously maintain the membrane separation performance over a long period of time while eliminating clogging of the separation membrane, various membrane separation operating techniques have been developed. Examples thereof include backwashing of passing permeated liquid, water, etc. from the permeation side to the raw water side to push out a substance stuck in a membrane pore or on the membrane surface; air washing of feeding a gas from the lower part of a hollow-fiber membrane module and physically cleaning the hollow-fiber membrane (i.e., a separation membrane in hollow fiber form) by shaking the membrane (see, for example, Patent Document 1); and a flushing method of flowing raw water or a chemical solution at a high linear velocity to a membrane surface on the raw water side of a hollow-fiber membrane (see, for example, Patent Document 2).
Patent Document 3 discloses a method for producing a hollow-fiber membrane by discharging and solidifying a polyvinylidene fluoride-based resin solution containing a polyvinylidene-based resin and a poor solvent for the resin, of which temperature is not less than the phase separation temperature, into a cooling bath at not more than the phase separation temperature. In Patent Document 3, it is stated that a hollow-fiber membrane having a spherical structure is obtained by the method above and the hollow-fiber membrane has high strength.